Power factor relates generally to the efficiency of a system in using power and is preferably maintained at a high level (i.e., 0.9 or higher) to minimize losses. In particular, power factor defines the relationship between the actual/active or real power being used by the system (i.e., measured in kilo-Watts (KW)) and the total power supplied and available to the system. Further, total power is defined by apparent power (i.e., measured in volt-amps (VA)), which includes a non-working component (i.e., reactive power (kVAR)). Thus, it is desirable to reduce, as much as possible, the reactive power component. Reactive power is generally not useful power, and typically provides for sustaining the electromagnetic field in systems.
Specifically, power factor is a value between 0 and 1, and represents the amount of power actually being used (i.e., real power divided by apparent power) by a device or system. A high power factor indicates that a system is using power efficiently, while a low power factor indicates that a system in using power less efficiently. Thus, when the power factor is 1, real power and apparent power are equal, with the system using power at 100% efficiency. However, when the current from a power source includes harmonics, or when it is not in phase with the voltage (e.g., reactive device), the power factor of the system is reduced (i.e., less than 1), thus indicating a less efficient system.
For example, reactive power may be caused by a phase shift between AC current and voltage in inductors and capacitors within a system. With respect to inductors causing phase shift, current is said to lag behind voltage, and in capacitors causing phase shift, current is said to lead voltage. Typically, when inductive loads cause lagging in a system, appropriate capacitors are used to correct and offset the lagging effect (i.e., increase power factor).
Depending upon the power supplier, the cost of receiving power may increase if power factor is not sustained at a specific minimum level (i.e., 0.9). Further, because power factor represents power that could be used, but is not, increasing the use of the available power will reduce overall cost. Additionally, larger wiring and transformers may be needed when power factor is low. Thus, a low power factor may have numerous negative effects on different aspects of a system.
Further, government regulations for certain power conversion equipment require high power factors (i.e., above 0.9). Additionally, regulations for semiconductor processing equipment also require sustaining power sources through line voltage dropouts, which further require energy storage elements at the power supply input that may need power correction. By way of example, semiconductor manufacturers frequently require that power supplies meet the standard SEMI F-47 specification in order to protect the integrated circuit fabrication process from voltage dropouts. The specification is summarized in part in the table below.
VOLTAGE SAGVOLTAGE SAG DURATIONPercent (%) ofCycles atEquipmentSecond(s)Milliseconds(ms)Cycles at 60 hz50 hzNominal Voltage<0.05s<50ms<3cycles<2.5cyclesNot specified0.05 to 0.2s50 to 200ms3 to 12cycles2.5 to 10cycles50%0.2 to 0.5s200 to 500ms12 to 30cycles10 to 25cycles70%0.5 to 1.0s500 to 1000ms30 to 60cycles25 to 50cycles80%>1.0s>1000ms>60cycles>50cyclesNot Specified
Power factor correction (PFC) devices are known that provide for maintaining the power factor at higher levels, typically above 0.85. Both passive and active devices have been developed in an attempt to increase and maintain the power factor of a system at a high level. In a passive approach, an inductor is provided at the input of the circuit or system, usually ahead of an electrolytic capacitor bank. This helps to reduce harmonic distortion within the system and allows equipment to obtain a power factor of between about 0.8 and 0.9. However, the inductance value required increases exponentially with the power factor improvement desired. When a power factor above 0.9-0.95 is desired or required, the size and weight of the inductor becomes prohibitive.
Active approaches include boost, buck, or flyback converters in connection with monitoring devices that monitor various variables within the system. In particular, the input voltage, the output voltage and current at the input of the system are monitored to maintain the power factor. However, although these PFC devices provide higher power factors (i.e., above 0.9), these devices are complex, resulting in their size and cost increasing significantly. Reliability of the system also may be reduced with the addition of these complex control components.
Thus, there exists a need for a system for maintaining power during a line voltage dropout that is less complex in design, lower in cost and that provides a higher power factor.